The proposed case-control study will compare 80 frequent marijuana users (5 times weekly or more) with 80 moderate users (2 or fewer times weekly) among undergraduates at two colleges. The study instrument will assess 1) basic demographic variables; 2) history of marijuana use; 3) history of use of other drugs; 4) current and past DSM-III-R Axis I and Axis II psychiatric disorders as determined by the Structured Clinical Interview for DSM-III-R; 5) neuropsychological measures as assessed by a battery of basic tests, together with the Rand Mental Health Inventory; and 6) students' subjective descriptions of possible residual marijuana effects. Subjects will be required to stay at McLean Hospital under observation for 24 hours prior to testing to ensure that they do not use marijuana during this period. Urine will be obtained at the time of interview to screen for tetrahydrocannabinol and for other drugs of abuse. Data analysis will compare the two groups on all of the above variables. To test for the possible influence of factors other than current frequency of marijuana use, within-group comparisons will compare students 1) with and without current and/or past Axis I or Axis II disorders, 2) with and without current and/or past use of other drug categories (by verbal report and/or urine screen), and in the case of moderate users, those 3) with and without a past history of frequent marijuana use. Variables other than frequency of marijuana use that appear associated with neuropsychiatric findings will be entered into a logistic regression analysis in order to determine whether they are significantly associated with neuropsychiatric abnormalities independently of marijuana usage. It is hypothesized, on the basis of a pilot study, that marked and significant differences will distinguish frequent from moderate users, and that many of these differences are directly attributable to marijuana use, rather than to pre-existing psychological differences between groups or to effects of other drug use.